The bones and connective tissue of an adult human spinal column consists of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex that consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than twenty bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
The spinal column is highly complex in that it includes these more than twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art that achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.
Referring now to FIGS. 1-2, in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages 1 generally comprise tubular metal body 2 having an external surface threading 3. They are inserted transverse to the axis of the spine 4, into preformed cylindrical holes at the junction of adjacent vertebral bodies (in FIG. 2 the pair of cages 1 are inserted between the. fifth lumbar vertebra (L5) and the top of the sacrum (S1)). Two cages 1 are generally inserted side by side with the external threading 4 tapping into the lower surface of the vertebral bone above (L5), and the first surface of the vertebral bone (S1) below. The cages 1 include holes 5 through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior 6 of the cage 1 to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage 1.
These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient's spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.
It is, therefore, an object of the invention to provide an intervertebral spacer that stabilizes the spine without promoting a bone fusion across the intervertebral space.
It is further an object of the invention to provide an implant device that stabilizes the spine while still permitting normal motion.
It is further an object of the invention to provide a device for implantation into the intervertebral space that does not promote the abnormal distribution of biomechanical stresses on the patient's spine.
It is further an object of the invention to provide an artificial disc that provides free rotation of the baseplates relative to one another.
It is further an object of the invention to provide an artificial disc that supports compression loads.
It is further an object of the invention to provide an artificial disc that permits the baseplates to axially compress toward one another under a compressive load.
It is further an object of the invention to provide an artificial disc that permits the baseplates to axially compress toward one another under a compressive load and restore to their original uncompressed relative positions when the compressive load is relieved.
It is further an object of the invention to provide an artificial disc that prevents lateral translation of the baseplates relative to one another.
It is further an object of the invention to provide an artificial disc that provides a centroid of motion centrally located within the intervertebral space.
It is further an object of the invention to provide artificial intervertebral disc baseplates having outwardly facing surfaces that conform to the concave surface of adjacent vertebral bodies.
It is a further object of the present invention to provide a disc replacement device having a first element for seating against a lower endplate surface of a superior vertebral body and a second element for seating against an first end plate surface of an inferior vertebral body, said baseplates having disposed therebetween a partial spherical member having a large radius disposed in a complementary concavity such that said baseplates are articulatable against one another.
It is yet a further object of the present invention to provide a disc replacement device that is resistant to point loading and fatigue failure.
It is still a further object of the present invention to provide a disc replacement device employing ball and socket type articulation using a partial spherical member wherein said partial spherical member is not captured.
Other objects of the invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.